Heat-sensitive recording materials are well known, which use heat to obtain recorded images by utilizing a color development reaction between a leuco dye and a developer. Because such heat-sensitive recording materials are relatively inexpensive, and recording devices therefor can be relatively compact and easily maintained, they have been widely used, not only as recording materials for the output of facsimile machines and various computers, printers of scientific measuring instruments and the like, but also as recording materials for various printers for POS labels, ATMs, CAD, handy terminals, various ticket forms, and the like.
In order to improve the recording sensitivity and image quality of a heat-sensitive recording material, it is known to provide, between a support and a heat-sensitive layer, an undercoat layer in which a pigment and a binder are contained so that voids are formed therein to make it porous or bulky and to thereby impart thermal insulation properties. For example, it has been disclosed that, in order to obtain a uniform and stable undercoat layer structure, an undercoat layer coating composition having a specific viscosity may be applied by blade coating (patent document 1). It has also been disclosed that, in order to enhance the image quality of a heat-sensitive paper, variations in the thickness of an undercoat layer may be controlled within a specific range (patent document 2). It has been further disclosed that, in order to lower the coefficient of static friction of the surface, an undercoat layer having two or more layers may be formed by blade coating (patent document 3).
However, with recent increases in the speed of printing, the demand has grown for heat-sensitive recording materials having higher sensitivity and better image quality, and accordingly, it has been difficult to achieve sufficient quality simply by using a highly flat base paper or providing only an undercoat layer.
In methods for providing an undercoat layer, usually, a larger coating amount leads to an undercoat layer having better thermal insulation, thereby improving recording sensitivity. However, when the coating amount is larger, the formation of a uniform coating surface is less easy, making it difficult to make the subsequently formed heat-sensitive recording layer and protective layer uniform. As a result, recording sensitivity and image quality are lowered, and the barrier properties of the protective layer are impaired.
Further, in the heat-sensitive recording material market, which is now growing into a mature market, production costs have become an important issue. Accordingly, it would be difficult to employ, for actual production, coating techniques having low productivity or inducing coating defects, even if high-quality products can be thereby obtained.    [Patent document 1] Japanese Unexamined Patent Publication No. 1992-290789    [Patent document 2] Japanese Unexamined Patent Publication No. 2004-122483    [Patent document 3] Japanese Unexamined Patent Publication No. 2005-103864